SELECT 
  variant_value 
FROM 
  jmj_custom_variant_total 
WHERE 
  product_id = 818 
  AND featureid = 371 
  AND variantid = 2841

Query time 0.00396

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1620.60"
    },
    "table": {
      "table_name": "jmj_custom_variant_total",
      "access_type": "ALL",
      "rows_examined_per_scan": 7978,
      "rows_produced_per_join": 7,
      "filtered": "0.10",
      "cost_info": {
        "read_cost": "1619.00",
        "eval_cost": "1.60",
        "prefix_cost": "1620.60",
        "data_read_per_join": "191"
      },
      "used_columns": [
        "product_id",
        "featureid",
        "variantid",
        "variant_value"
      ],
      "attached_condition": "((`jmj_test`.`jmj_custom_variant_total`.`variantid` = 2841) and (`jmj_test`.`jmj_custom_variant_total`.`featureid` = 371) and (`jmj_test`.`jmj_custom_variant_total`.`product_id` = 818))"
    }
  }
}